Superabsorbent polymers (SAPs) refer to a synthetic polymer material having a function capable of absorbing 500 to 1000 times of moisture in a weight with respect to the polymer weight itself, and have started to be commercialized as sanitary products, and are widely used currently as soil repair materials for gardening, water stop materials for civil engineering and construction, sheets for raising seedlings, freshkeeping materials in food distribution fields, and materials for fomentation and the like in addition to personal hygiene items such as paper diapers for children and sanitary napkins.
In such superabsorbent polymer preparation processes, water is a polymerization medium and the use is diverse such as facilitating crosslinking liquid dispersion in a surface crosslinking process. In addition, residual moisture in a final product performs a role of an antistatic agent for polymers and a plasticizer, suppresses formation of very small superabsorbent polymer dusts in application processes, and prevents milling of SAP particles. However, when water is added to a superabsorbent polymer even in a small amount, stickiness on a surface of the polymer generally increases due to water absorbed on the surface, and irreversible agglomeration between the superabsorbent polymer particles occurs. Such a stickiness increase and agglomeration reduce processability such as a load increase in preparation and application processes, which resultantly causes a particle size increase, physical property decline and productivity decline of superabsorbent polymers. So far, researches on superabsorbent polymers have focused on polymerization processes and absorption capacity enhancement obtained therethrough, and surface crosslink for increasing surface properties or absorption under pressure of the superabsorbent polymers, and researches on the changes in surface properties have been carried out for enhancing permeability, or solving some problems such as preventing hardening of superabsorbent polymers when being stored (anti-caking).
Specifically, superabsorbent polymers have a crosslinked three-dimensional network structure and are swollen by water, but have a structure insoluble in water, and accordingly, core/shell structure technologies increasing crosslink density on the surface have been developed for high absorption under pressure and permeability properties of superabsorbent polymers. In such technologies, final physical properties are determined depending on various conditions such as crosslink density adjustment, uniform shell formation and shell thickness adjustment. With the recent development of pulpless diapers, physical properties of superabsorbent polymers themselves are directly connected to physical properties of diapers, and importance of superabsorbent polymers has gradually grown.
Particularly, a surface crosslinking agent is dissolved in water to be used in a surface crosslinking process of a superabsorbent polymer, and although the water used herein is a solvent enabling the crosslinking agent to be absorbed on the surface of the superabsorbent polymer, it may increase stickiness on the superabsorbent polymer surface by inducing non-uniform penetration and become a reason for partial agglomeration. Accordingly, for solving such a problem, an organic solvent is excessively used for controlling the absorbed amount and facilitating dispersion of the crosslinking agent into more particles, and this environmentally requires additional processes such as removing the residual organic solvent, and particularly, is not suitable to be used for diapers for newborn babies.
In this regard, Korean Patent Application Laid-Open Publication No. 2014-0038998 discloses a method for preparing absorbent polymer powder, however, the absorbent polymer powder has a limit not only in absorption under pressure and permeability properties, but in reducing an amount of a surface crosslinking agent used. Consequently, the development of a method for preparing a superabsorbent polymer having excellent absorption under pressure and permeability properties while reducing an amount of a surface crosslinking agent used for enhancing physical properties of the superabsorbent polymer has been required.